3,778 research outputs found
On Coulomb drag in double layer systems
We argue, for a wide class of systems including graphene, that in the low
temperature, high density, large separation and strong screening limits the
drag resistivity behaves as d^{-4}, where d is the separation between the two
layers. The results are independent of the energy dispersion relation, the
dependence on momentum of the transport time, and the wave function structure
factors. We discuss how a correct treatment of the electron-electron
interactions in an inhomogeneous dielectric background changes the theoretical
analysis of the experimental drag results of Ref. [1]. We find that a
quantitative understanding of the available experimental data [1] for drag in
graphene is lacking.Comment: http://iopscience.iop.org/0953-8984/24/33/335602
Experimental Demonstration of Optimal Unambiguous State Discrimination
We present the first full demonstration of unambiguous state discrimination
between non-orthogonal quantum states. Using a novel free space interferometer
we have realised the optimum quantum measurement scheme for two non-orthogonal
states of light, known as the Ivanovic-Dieks-Peres (IDP) measurement. We have
for the first time gained access to all three possible outcomes of this
measurement. All aspects of this generalised measurement scheme, including its
superiority over a standard von Neumann measurement, have been demonstrated
within 1.5% of the IDP predictions
Effect of Holstein phonons on the electronic properties of graphene
We obtain the self-energy of the electronic propagator due to the presence of
Holstein polarons within the first Born approximation. This leads to a
renormalization of the Fermi velocity of one percent. We further compute the
optical conductivity of the system at the Dirac point and at finite doping
within the Kubo-formula. We argue that the effects due to Holstein phonons are
negligible and that the Boltzmann approach which does not include inter-band
transition and can thus not treat optical phonons due to their high energy of
eV, remains valid.Comment: 13 pages, 4 figure
Relativistic Doppler effect in quantum communication
When an electromagnetic signal propagates in vacuo, a polarization detector
cannot be rigorously perpendicular to the wave vector because of diffraction
effects. The vacuum behaves as a noisy channel, even if the detectors are
perfect. The ``noise'' can however be reduced and nearly cancelled by a
relative motion of the observer toward the source. The standard definition of a
reduced density matrix fails for photon polarization, because the
transversality condition behaves like a superselection rule. We can however
define an effective reduced density matrix which corresponds to a restricted
class of positive operator-valued measures. There are no pure photon qubits,
and no exactly orthogonal qubit states.Comment: 10 pages LaTe
Chaotic Evolution in Quantum Mechanics
A quantum system is described, whose wave function has a complexity which
increases exponentially with time. Namely, for any fixed orthonormal basis, the
number of components required for an accurate representation of the wave
function increases exponentially.Comment: 8 pages (LaTeX 16 kB, followed by PostScript 2 kB for figure
Non-linear operations in quantum information theory
Quantum information theory is used to analize various non-linear operations
on quantum states. The universal disentanglement machine is shown to be
impossible, and partial (negative) results are obtained in the state-dependent
case. The efficiency of the transformation of non-orthogonal states into
orthogonal ones is discussed.Comment: 11 pages, LaTeX, 3 figures on separate page
Nonclassical correlation in a multipartite quantum system: two measures and evaluation
There is a commonly recognized paradigm in which a multipartite quantum
system described by a density matrix having no product eigenbasis is considered
to possess nonclassical correlation. Supporting this paradigm, we define two
entropic measures of nonclassical correlation of a multipartite quantum system.
One is defined as the minimum uncertainty about a joint system after we collect
outcomes of particular local measurements. The other is defined by taking the
maximum over all local systems about the minimum distance between a genuine set
and a mimic set of eigenvalues of a reduced density matrix of a local system.
The latter measure is based on an artificial game to create mimic eigenvalues
of a reduced density matrix of a local system from eigenvalues of a density
matrix of a global system. Numerical computation of these measures for several
examples is performed.Comment: v1: 10 pages, 8 figures, IOPART, v2: introduction modified, figure 7
replaced, v3: 10 pages, 10 figures, RevTeX4, major revision with an
additional measure introduced, title changed (previous title: Non-classical
correlation in a multi-partite quantum system reconsidered), to appear in
Phys. Rev.
Scattering by linear defects in graphene: a tight-binding approach
We develop an analytical scattering formalism for computing the transmittance through periodic defect lines within the tight-binding model of graphene. We first illustrate the method with a relatively simple case, the pentagon-only defect line. Afterwards, more complex defect lines are treated, namely the zz(558) and the zz(5757) ones. The formalism developed, only uses simple tight-binding concepts, reducing the problem to matrix manipulations which can be easily worked out by any computational algebraic calculator.JNBR was supported by Fundacao para a Ciencia e a Tecnologia (FCT) through Grant No. SFRH/BD/44456/2008. NMRP was supported by Fundos FEDER through the Programa Operacional Factores de Competitividade-COMPETE and by FCT under project no. PEst-C/FIS/UI0607/2011. NMRP acknowledges both the hospitality and the funding from the Graphene Research Centre at the National University of Singapore, where this work was completed
Non-full rank bound entangled states satisfying the range criterion
A systematic method for generating bound entangled states in any bipartite
system, with ranks ranging from five to full rank, is presented. These states
are constructed by mixing separable states with UPB (Unextendible Product
Basis) - generated PPT bound entangled states. A subset of this class of PPT
bound entangled states, having less than full rank, is shown to satisfy the
range criterion [Phys. Lett. A, vol. 232 (1997) 333].Comment: 6 pages, Latex. Minor corrections and additions. More references
added. Accepted for publication in Phys. Rev.
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